The present invention relates to a semiconductor device, and more particularly to an improvement on the hermetic seal of a semiconductor element.
Generally, a power semiconductor element is contained or encapsulated in a vessel such as a ceramic seal, a glass seal or the like. This is because the electrical characteristics of the PN junction are easily affected by the atmosphere.
In a conventional high-power semiconductor device, metal electrodes are provided on both sides of a semiconductor element, ends of an insulating cylinder of ceramics, glass or the like is disposed to surround or be wrapped around the semiconductor element and the metal electrodes and ends of the insulator cylinder are brazed to the peripheries of the electrodes by metal flanges to obtain hermetic seal. But, the semiconductor device formed as described above is expensive because the hermeticallysealing vessel is costly.
One consideration to be made in connection with the desired hermetic seal is that in recent years glass films are formed at the PN junction of a semiconductor element and the stability of the electrical characteristics against the atmosphere have been improved. As a result, the requirement on the hermetic seal has been alleviated. An example of simplified vessel which is formed of less expensive materials and can be easily assembled is shown in FIG. 1.
The semiconductor device shown in FIG. 1 comprises a semiconductor element 1 comprising a silicon wafer having a PN junction and molybdenum disks brazed on both sides of the silicon wafer. A cathode electrode 2 and an anode electrode 3 are formed of copper and plated with nickel, and are provided on both sides of the semiconductor element 1. The electrodes 2 and 3 have flanges 2a and 3a, which are provided with dents 2b and 3b. A sealing ring 4 formed of a resilient insulator has grooves 4a on its inner periphery, and the flanges 2a and 3a are fitted in the grooves 4a. Thus, the device is so assembled that the insulating ring 4 is disposed to surround the semiconductor element 1. Radiator fins, not shown, are thereafter pressed on the electrodes 2 and 3 to achieve contact between the radiator fins and the electrodes 2 and 3, and between the electrodes 2 and 3 and the semiconductor element, thereby to obtain thermal and electrical connection. The hermetic seal is obtained between the flanges 2a and 3a of the electrodes 2 and 3, and the grooves 4a of the resilient sealing ring 4.
With the above-described semiconductor device, the hermetic seal is achieved only when the radiator fins are mounted. If the period until the radiator fins are mounted is long, the molybdenum electrodes of the semiconductor element 1 may be corroded by the moisture in the atmosphere, so that the contact resistances with the electrodes 2 and 3 are increased and the electrical and the thermal characteristics are degraded. To prevent the corrosion, gold may be plated or gold foils may be cladded on the molybdenum electrodes of the semiconductor element 1. Use of gold or gold foils however increases the cost of the device. If the gold or gold foils are not used, the semiconductor device cannot be stored for a long time, and the process control is difficult. Moreover, the device is subject to undesirable disintegration by accidental application of a small impact, which makes the handling of the device inconvenient. Thus, there is a restriction in this respect on the users of the semiconductor device.